1. Field of the Invention
The present invention relates to a method and apparatus for use in a measuring and/or control device with a sensor based on eddy current induction moving over a test piece, and particularly to such apparatus for determining the edges of the test piece.
2. Related Art
The invention has been developed particularly for application in conjunction with measuring apparatus of the kind which incorporate proximity sensors based on eddy current induction and intended for measuring cracks present in metal objects having relatively broad surfaces, such as metal sheet, cylinders or the like. One field in which the invention finds particular applicability is that of continuous casting, in which the solidifying steel ingot is advanced in a continuous length. The continuous metal ingot is then cut into billets, which are subsequently treated by rolling or like processes. Cracks frequenty occur in the cast material. Consequently, a crack detecting apparatus incorporating proximity sensors based on eddy current induction has been developed for detecting the presence of surface cracks in the material. This apparatus is described and illustrated in Swedish Patent Applications Nos. 7613708-2, 8206678-8 and 8302738-3.
The cracks are located by passing a sensor head provided with eddy current inducing proximity sensors across the surface of a test piece, e.g. the continuous ingot, to be examined for cracks. The test piece is advanced continuously in a conventional manner, essentially transversely to the sweep direction of the head.
The width of continuous cast test pieces can vary considerably, and may even vary during one and the same casting operation. One problem which often occurs in continuous casting processes is that of continuously defining the width of the casting, e.g. the ingot, in a reliable manner. Hitherto, no apparatus which meets with complete satisfaction has been proposed for measuring or defining the width of castings, particularly hot continuous cast ingots.
One problem encountered with crack measuring apparatus using proximity sensors based on eddy current inducement is one of controlling the turning point of a sensor sweep, or scan, with the aid of the momentary location of an edge of a test piece. It is true that separate edge detecting units can be placed in the close proximity of the position at which the sensor passes out over the edge of the test piece during a sweep or scan, but the use of additional measuring systems is always to be avoided as far as possible, since additional systems increase costs, complicate the technical solution, and require space.